1. Field of the Invention
The present invention relates to wireless communications systems. More particularly, the present invention relates to on-frequency repeaters, whose input and output signals have the same frequency, which are used to extend base station coverage in a cellular environment.
2. Description of the Prior Art and Related Information
In a wireless communication system, a mobile unit such as a cellular phone transmits and receives radio frequency (RF) signals to and from cell site base stations. Multiple users can share a common communication medium through technologies such as code division multiple access (CDMA), time division multiple access (TDMA), and global system for mobile communications (GSM).
Fading, caused by attenuation, delay, and phase shift of signals during propagation, particularly during multipath propagation, limits the base station coverage.
Repeaters are usually included in the system to receive, filter, and re-transmit signals of interest at a higher power level, thus extending base station coverage. The signals of interest received by a repeater may be from a base station, a mobile unit, or from another repeater. The forward gain of a repeater is an important characteristic of the repeater as it determines the amplified power level of the re-transmitted signals.
On-frequency repeaters have input and output signals at the same frequency. Thus, the coupling between the input and output signals is an issue. If the product of the forward gain of the repeater and the coupling between the input and output antennas is too high, the repeater will oscillate. In general, this is undesirable.
To avoid oscillations, the isolation between the input and output antennas must exceed the forward gain of the repeater. As a result, the forward gain of the repeater is often limited, typically, by the feedback associated with coupling between the input and output antennas. Although the antenna radiation patterns are designed to limit the coupling, in practice, some output power is still coupled to the input after various delays.
Accordingly, a need presently exists for a system and method for addressing this problem of repeater antenna coupling so that the forward gain of the repeater can be increased.